Interfacial Electronic Modulation of Dual-Monodispersed Pt–Ni3S2 as Efficacious Bi-Functional Electrocatalysts for Concurrent H2 Evolution and Methanol Selective Oxidation

Highlights The well-conceived Pt–Ni3S2 heteronanocrystals with dual-monodispersed characteristics are synthesized through interfacial electronic modulation. The asymmetrical charge distribution at Pt–Ni3S2 hetero-interface results in the formation of high-valent Ni sites and negatively-charged Ptδ−. It eventually accelerates water dissociation and achieves the steady concurrent generation of value-added formate and hydrogen. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01282-4.

temperature.The product was dissolved in toluene and the solution was centrifuged at 12000 rpm during 10 minutes for nanoparticles separation.Finally, the as-synthesized Ni3S2 nanocrystals were thoroughly purified by multiple precipitation and re-dispersion steps using toluene and isopropanol.

Preparation of Working Electrode
The Pt-Ni3S2 catalyst modified carbon cloth is employed as both cathode and anode electrodes.Prior to the modification, carbon cloth (CC) was cut into 0.5 cm × 1.5 cm rectangular pieces, and then they were rinsed with water and ethanol thoroughly under sonication to remove residual organic species.For electrode modification, we use fresh suspensions to prepare ink by the following steps: the toluene suspension (1 mL) was separately dissolved in isopropanol and ethanol, and the solution was centrifuged at 10000 rpm during 5 minutes to precipitate metal complexes.Then the as-synthesized Pt-Ni3S2 catalyst and 5 mg carbon black was finally dispersed in ethanol (200 μL), and 25 μL 5 wt % Nafion solution.The mixed solution was followed by ultrasonication for 30 min to obtain a homogeneous catalyst suspension.Then 25 μL catalyst ink was dropped on the carbon cloth (loading area 0.5 cm × 0.5 cm) yielding a mass loading of 1.5 mg cm −2 .The catalyst modified electrode was dried at 60 o C before electrochemical measurements.At least three identical electrodes for each electrocatalysts were made for the repeatability test.

S1.4 Reference Electrode (RE) Calibration
We used Ag/AgCl as the reference electrode for all measurements.The calibration was performed in the high purity hydrogen saturated electrolyte with a Pt foil as the working electrode and counter electrode.CV were run at a scan rate of 1 mV s -1 in 1 M KOH with/without 1 M methanol, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions.The calibrated potentials measured against Ag/AgCl with RE calibration was convert to the reversible hydrogen electrode was calculated as follow: E (RHE) = E (Ag/AgCl) + 1.0205 V (Fig. S5)[1, 2].

S1.5 Catalyst Characterization
The Pt-Ni3S2 catalyst were characterized by field emission transmission electron microscopy (FETEM) under the acceleration voltage of 200 kV and equipped with a EMSIS Xarosa CCD camera and Oxford INCA (Aztec) EDS detector at 20 kV with the potential of performing elemental analysis on the mode of dark field.To prepare the TEM specimens, one drop of the suspension was placed on a carbon film supported molybdenum grid and allowed to dry in air before the specimens were transferred into the microscope.
Crystallographic and purity information on Pt-Ni3S2 catalyst were obtained using powder XRD.To analyze these materials, the as-synthesized samples (dispersed in isopropanol) after centrifugation were later air-dried upon deposition onto glass slides.Diffraction patterns of these materials were collected using a powder diffractometer (RIGAKU Smartlab) operating in the reflection mode equipped with a Ni filter and a Cu Kα radiation (λ = 1.5406Å) source with the accelerating voltage of 40 kV and the applied current of 200 mA.The scanning is performed at 10 o min −1 from 10 ~ 80 o with a step size of 0.02 o .X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250Xi spectrometer employing a monochromatic Al Kα X-ray source (hν = 1486.8eV) and 500 μm test spot area under 15 kV test tube voltage, 10 mA tube current, and 2 × 10 −9 mbar room floor vacuum.The takeoff angle for the collection of photoelectrons was 90 o from the surface normal.Survey spectra were recorded on all samples followed by high resolution XPS spectra for Ni 2p, S 2p, Pt 2p and O 1s spectral regions.All the peaks were calibrated with C 1s spectrum at binding energy of 284.8 eV.
Ni K-edge X-ray absorption fine structure (XAFS) spectra of the Pt-Ni3S2 catalyst were recorded by synchrotron radiation light source at the VESPERS beamline of the Canadian Light Source (CLS) with medium energy range of 6-30 keV.The catalysts were dispersed on Kapton (polyimide) tape for XAFS measurements using a four element vortex detector in the transmission mode at room temperature.The Si (111) crystal monochromator was utilized to acquire the scan for X-ray spectra, and the data was collected in the total electron yield (TEY) mode by measuring the sample drain current.The XAFS spectra include X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS).Energy calibration was performed with a standard nickel foil by shifting all spectra to a glitch in the incident intensity.Each spectrum was mathematically processed by a standard normalization method to exclude the influence of sample thickness, absorber concentration, detector, and amplifier settings.The k3-weighted Fourier transformation of EXAFS (FT-EXAFS) and the k2weighted Wavelet transforms of the EXAFS (WT-EXAFS) spectra were made to obtain the information on local electronic structures and radial distribution environment of Ni atoms in Pt-Ni3S2 catalyst.

Identification and Quantification of Formate Product
The identification and quantification of formate product are conducted by Ion Chromatography (IC) and determined by calibration curve.A small amount of electrolyte was taken by syringe after anode-cathode interchangeable electrocatalysis under ISTEP, and the electrolyte was properly diluted and consequently analyzed by Ion Chromatography (IC) to detect the formate generation.The Ion Chromatography (IC) was carried out on a CIC-D120 ion chromatograph (Shenghan Chromatography Technology Co., Ltd, Qingdao, China) equipped with an SH-AC-3 type anionic column.An aqueous solution containing sodium carbonate (2.4 mmol L −1 ) and sodium bicarbonate (6.0 mmol L −1 ) is employed as the eluent.The measurement is conducted at a constant temperature of 35 o C with a flow rate of 1.0 mL min −1 .At least three accurate trace curves were collected for statistical analysis.
The identification and quantification of the formate products were determined by calibration curve by applying standard formate solutions with known concentrations of commercially purchased pure sodium formate (chromatographic pure) (Fig. S13a).
The Faradaic efficiency (FE) of formate generation was calculated using the following equation: where F is the Faraday constant (96485 C mol −1 ).
ωt(mg L −1 ) is the concentration of formed formate in the electrolyte, namely, the IC data (ppm).The unit of ppm here is mass(formate)/volume(solution).
V(L) is the total volume of the electrolyte.
Mformate(g mol −1 ) is the molecular weight of formate (HCOO − ) equal to 45.02 g mol −1 .I(A) is the current recorded by the electrochemical workstation in the ISTEP mode.

S1.7 Identification and Quantification of Gas Products
The generated H2 from the electrolyzer were determined by gas chromatography (5977B MSD, Agilent Technologies) with a thermal conductivity detector (TCD).Argon (purity: 99.999%) was used as a carrier gas with a constant flow rate of 20 mL min −1 .A stable flow rate of mixed gas (the gaseous products and the carrier gas) was achieved after a period of electrocatalytic reaction, which gave the accurate GC traces for measurements.The GC sampling was conducted at 1, 2, 3, 4, 5 and 6 hours of electrocatalytic reaction.The identification and quantification of the H2 products were determined by calibration curve by applying commercial standard H2 gas with known concentrations.
The generation rate (mol s −1 ) of H2 was calculated using the following equation: where v(vol ratio) is the volume concentration of H2 in the exhaust gas from the electrolyzer, namely, the GC data (volume ppm).The unit of ppm here is volume(H2)/volume(total).
V(m 3 s −1 ) is the gas flow rate measured by a flow meter at room temperature and under ambient pressure.
The Faradaic efficiency (FE) of H2 production was calculated using the following equation: where F is the Faraday constant (96485 C mol −1 ).
v(vol ratio) is the volume concentration of H2 in the exhaust gas from the electrolyzer, namely, the GC data (volume ppm).The unit of ppm here is volume(H2)/volume(total).
V(m 3 s −1 ) is the gas flow rate measured by a flow meter at room temperature and under ambient pressure.I(A) is the current recorded by the electrochemical workstation in the ISTEP mode.The values of TOF were calculated by assuming that all metal atoms are involved in the catalytic processes, which all represent the lowest limits of the models: TOF=j*S/zFn where j (mA cm -2 ) is the as-measured current density at various potentials, S (cm -2 ) represents the surface area of the glassy carbon disk, the number z means a four-electron transfer during the formation of one mole of HCOOH for MOR (two-electron transfer during the formation of one mole of H2 for HER), F is the Faraday's constant (96485.3C mol -1 ), and n is the moles of Ni atoms on the electrode which can be calculated by the loading weight and the molecular weight of the coated catalysts.

Scheme S1
Scheme S1 Diagram of the synthesis of Ni3S2 and Pt-Ni3S2

Fig. S2
Fig.S2TEM image and size distribution plot clearly shows a dual-monodispersed system with a particle size of 9.61 ± 0.31 nm of Pt-Ni3S2 nanocrystals, in which, monodisperse Pt particles are only 2.08 ± 0.04 nm in size

Fig. S10
Fig.S10The decreased oxidative potentials of methanol upgrading reaction using Pt-Ni3S2/CC electrode at certain current densities compared with Ni3S2/CC electrode

Fig. S15
Fig. S15 High-resolution XPS spectra.The comparison of fresh and used Pt-Ni3S2 nanoheterostructures in (a) Ni 2p, (b) Pt 4f, (c) S 2p and (d) O 1s regions after MOR stability tests by chronoamperometry (I-t) at 2.12 V (vs.RHE) with an initial current density of ~1000 mA cm -2 for 72 h

Table S1
Element content analysis of Pt-Ni3S2 via ICP-MS.

Table S2
Fitting impedance parameters of various catalysts for MOR

Table S3
Fitting impedance parameters of various catalysts for HER